U.S. Pat. No. 3,031,433, U.S. Pat. No. 4,049,638, U.S. Pat. No. 5,519,097, and EP 94/01998 disclose single stage pipeline polyamide reactors operating at pressures above the vapor pressure of aqueous caprolactam feed solutions, such that only a single liquid phase will be present. None of these references disclose operation of a reactor system comprising a reactor, a flasher and a separator, wherein the reactor is operated below the system vapor pressure so that boiling occurs and results in operation in a two phase (gas/liquid) region.
U.S. Pat. No. 4,539,391 discloses first stage pipeline reactors operating at reactor pressures under 10 bar (145 psig, 130.3 psig, 1000 kPa) and reactor residence times of under 10 minutes. U.S. Pat. No. 4,539,391 does not disclose operation at pressures between 10 bar (145 psig, 130.3 psig, 1000 kPa) and the system vapor pressure and the resulting benefit of providing sufficient pressure to drive the flasher stage that follows the reactor, nor the advantage of longer residence times between 10 and 180 minutes to allow for higher conversions of monomer to prepolymer with minimum cyclic prepolymer formation. The '391 patent further discloses that following the pipeline reactor is a high pressure separator operating at the same pressure as the reactor. The prepolymer exiting from this separator has a high level of extractable impurities (10 to 11% by weight).
The process of the present invention differs from U.S. Pat. No. 4,539,391 in that a low holdup time flasher follows the reactor stage which lets down the pressure and strips off the extractable impurities. Further, in the process of the present invention, a very low holdup time 1 atm.(101 kPa) separator follows the flasher which produces prepolymer with low levels of extractable impurities. The '391 patent discloses the need for water extraction of extractable impurities. The product of the process of the present invention does not require water extraction.
DE 196 35 077 A1 discloses a process for the continuous preparation of polyamides from omega-aminoalkylnitriles, specifically 6-aminocapronitrile, comprising three process steps, the first of which is a single phase reaction of the omega aminoalkylnitrile in water.
WO 98/08889 discloses a multi stage process for producing polyamides from aminonitriles under specified temperature and pressure conditions.
U.S. Pat. No. 5,596,070 discloses a process for the preparation of high molecular weight polyamides from cyano or amino containing monomers wherein the final solid state polymerization step is carried out in the presence of an inert gas which contains at least 50% by volume of superheated steam